XFEM-based numerical modeling of well performance considering proppant transport, embedment, crushing and rock creep in shale gas reservoirs

Predicting the production rate and estimating the ultimate recovery are of great significance for the optimization of shale gas development. One possible reason for production decline is the width reduction of propped fractures caused by proppant deformation, embedment, crushing, and viscoelasticity of the reservoir. In order to obtain more in-depth knowledge of how these factors influence the gas production, a novel approach is proposed to simulate the processes of both hydraulic fracturing and gas production using the extended finite element method (XFEM). On the one hand, a numerical model which involves the coupled processes of deformation of the porous rock medium, fluid flow and leak-off, crack propagation, and proppant transport is proposed to obtain the proppant distribution for the hydraulic fracturing simulation. On the other hand, a coupled rock deformation and two-phase flow model considering stress-dependent fracture conductivity, crushing of proppant grains, creep of surrounding rock, gas diffusion and desorption, and influence of natural fractures is developed for the flowback and gas production simulation. The fracture closure during the shut-in and flowback process of fracturing fluid is considered. In addition, the size effects of proppant grains are investigated by using Weibull theory. After validation of the proposed model through history-matching, the sensitivity analysis of the proposed model on gas production is conducted. This study reveals that the proppant size has a dominant influence. Although proppant with larger grain sizes leads to a more permeable propped fracture, it is found that only proppant grains of proper size, not too large or too small, has the potential to achieve economic gas production due to the combined effects of size effect of the grains and bridging out of the proppant. The second influential factor on the gas production is the proppant concentration which is followed by the viscosity of shale formation, the Young's modulus of proppant, and the Young's modulus of shale formation. This study leads to a better understanding of proppant-related mechanisms involved in hydraulic fracturing and gas production and provides an efficient numerical tool for the prediction of well performance in field development planning.

Automated summary

It is of great significance to predict the production rate and estimate the ultimate recovery for the optimization of shale gas development. A novel approach using XFEM is proposed to simulate the processes of hydraulic fracturing and gas production, revealing that proppant size has a dominant influence on gas production, followed by proppant concentration, shale formation viscosity, proppant Young's modulus, and shale formation Young's modulus.